The present invention relates to a catalyst based on cobalt, its preparation and its use in the Fischer-Tropsch process.
The Fischer-Tropsch process is a process well-known to experts in the field, which essentially consists in the hydrogenation of CO to give hydrocarbons. The reaction conditions are also described in literature.
Catalysts which can be used in the Fischer-Tropsch process generally consist of metals of group VIII supported on a carrier, preferably selected from alumina, silica, titania and relative mixtures.
All research projects on the Fischer-Tropsch process are being increasingly orientated towards a greater selectivity to C9+, particularly C22+ hydrocarbons, the latter also known as Fischer-Tropsch waxes.
Among these Fischer-Tropsch catalysts, Cobalt, which is particularly effective in directing the reaction towards the formation of waxes, is becoming more and more widely used.
A particular catalyst based on cobalt supported on alumina has now been found, which is more selective, with respect to normal cobalt catalysts, towards the formation of waxes in the Fischer-Tropsch process.
In accordance with this, the present invention relates to a catalyst which can be used in the Fischer-Tropsch process, essentially consisting of cobalt oxide supported on an inert carrier essentially consisting of alumina, characterized in that the above cobalt oxide essentially consists of crystals having an average size ranging from 20 to 80 xc3x85, preferably from 25 to 60 xc3x85, even more preferably from 30 to 40 xc3x85.
The above crystals of cobalt oxide can be optionally partly doped with Al atoms.
With respect to catalysts known in the art, the catalyst of the present invention has the characteristic of consisting of crystals with much lower dimensions, 20-80 xc3x85 for the catalyst of the present invention against the 120-180 xc3x85 of a catalyst prepared according to the conventional techniques. This allows a better dispersion of the cobalt on the carrier, with a consequent better contact, in reaction phase, between the catalyst and reagents.
In addition to cobalt oxide, the catalyst of the present invention may optionally also contain, in a much lower quantity than the cobalt, metals normally known as promoters, such as Si, Zr, Ta, Zn, Sn, Mn, Ba, Ca, La, Ve, W. Promoters are used for improving the structural stability of the carrier itself.
One or more activity promoters with a different effect on the catalytic performances as described in the art (see for example B. Jager, R. Espinoza in xe2x80x9cCatalysis Todayxe2x80x9d, 23, 1995, 21-22), can also be optionally present together with the cobalt. For example, promoters such as K, Na, Mg, Sr, Cu, Mo, Ta, W and metals of group VIII essentially increase the activity. Ru, Zr, rare-earth oxides (REO), Ti increase the selectivity to high molecular weight hydrocarbons. Ru, REO, Re, Hf, Ce, U, Th favour the regenerability of cobalt catalysts.
As far as the alumina is concerned, this can have any phase form selected from eta, gamma, delta, theta, alpha and relative mixtures, in the presence of or without one or more structural stability promoters selected from those described above. In the preferred embodiment, the alumina is in xcex3 or xcex4 form, and relative mixtures.
The surface area of the alumina is that which is normal in catalytic carriers, i.e. from 20 to 300 m2/g, preferably from 50 to 200 m2/g (BET), whereas the average dimensions of the alumina itself range from 1 to 300 xcexcm.
The cobalt content of the catalyst of the present invention ranges from 2 to 50% by weight, preferably from 5 to 20% by weight, 100 being the total weight of the carrier and cobalt (plus possible promoters). When present, the promoters are in a quantity not higher than 20% by weight with respect to the cobalt, preferably 10% by weight.
Before being used in the Fischer-Tropsch process, the catalyst of the present invention should be activated by means of the usual procedures, for example by reduction of cobalt oxide to metallic cobalt in the presence of hydrogen.
In accordance with this, the present invention relates to a process for obtaining cobalt oxide supported on an inert carrier essentially consisting of alumina, the above cobalt oxide essentially consisting of crystals having an average size ranging from 20 to 80 xc3x85, which comprises the following steps:
1) preparation of an intermediate, supported on alumina, having general formula (I)
[Co2+1xe2x88x92xAl+3x(OH)2]x+[Anxe2x88x92x/n].mH2Oxe2x80x83xe2x80x83(I) 
xe2x80x83wherein x ranges from 0.2 to 0.4, preferably from 0.25 to 0.35, A is an anion, x/n is the number of anions necessary for neutralizing the positive charge, m ranges from 0 to 6, and is preferably 4;
2) calcination of the intermediate having general formula (I) with the formation of crystalline cobalt oxide.
With respect to the anion Anxe2x88x92, this can be indifferently selected from inorganic anions (for example Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, ClO4xe2x88x92, NO3xe2x88x92, OHxe2x88x92, IO3xe2x88x92, CO32xe2x88x92, SO42xe2x88x92, WO42xe2x88x92), hetero-polyacids (for example PMo12O403xe2x88x92, PW12O403xe2x88x92) organic acids (for example adipic, oxalic, succinic, malonic acid). In the preferred embodiment the anion Anxe2x88x92 is chosen from NO3xe2x88x92, OHxe2x88x92, CO32xe2x88x92. In an even more preferred embodiment Anxe2x88x92 is equal to CO3xe2x88x92xe2x88x92.
The compound having general formula (I) can be prepared according to various techniques known to experts in the field.
For example, the so-called precipitation technique can be used, according to which Co2+ and Al3+ are co-precipitated on alumina in the form of hydroxides. According to this technique, a solution of an Aluminum salt and a Cobalt salt, preferably an aqueous solution of the above salts, is dripped onto a suspension, preferably aqueous, of alumina. This operation must be effected maintaining the pH within a range of 6.6 to 7.2, preferably from 6.8 to 7.1, for example by the use of an aqueous solution of bicarbonate or soda. Alternatively, two separate solutions can be added; however, for the sake of simplicity, it is obviously preferable to use a single solution of the two salts. The compound having general formula (I) is recovered by means of filtration.
According to another less preferred embodiment, the so-called hydro-thermal technique can be used, which consists in treating freshly precipitated mixed cobalt and aluminum hydroxides, or mechanical mixtures of the oxides, with water.
The compound having general formula (I) can be amorphous or crystalline. The ratio between the amorphous part and the crystalline part can be modified using known techniques (for example by annealing). The crystalline part of the compound having general formula (I) has a structure similar to that of hydrotalcite.
Hydrotalcite is a mineral existing in nature and consists of an Al and Mg hydroxy-carbonate. A hydrotalcite-type system has an analogous structure, but contains different elements.
The group of hydrotalcites can be represented by the following formula:
[M2+1xe2x88x92xM+3x(OH)2]x+[Anxe2x88x92x/n].mH2O 
in the case of actual hydrotalcite M2+=Mg2+, M3+=Al3+ and Anxe2x88x92=CO32xe2x88x92.
One of the main characteristics of this group of compounds is the layered structure: layers of the brucite type, Mg(OH)2, or [M2+1xe2x88x92xM+3x(OH)2]x+, in which a part of the bivalent M2+ ions is substituted by trivalent M3+ ions, alternate with anionic layers associated with a varying content of water (Anxe2x88x92x/n].mH2O. The anionic layers balance the positive charge of the hydroxide layers, the latter linked to the presence of trivalent ions.
In general, M2+ and M3+ can be ions of a varying nature, the only requisite is that they are able to insert themselves in the cavities left by the hydroxyls in a compact configuration of the brucite type (more simply, they must have an ionic radius similar to that of Mg2+). The value of x in the structural formula ranges from 0.2 to 0.4, preferably from 0.25 to 0.35. Outside this range pure hydroxides or other compounds with a different structure can be obtained. Formulations of the hydrotalcite type have the following ratios M2+/M3+=2 and M2+/M3+=3, as limits in the composition, which do not imply differences in the lattice and structural parameters (only the internal distribution of the cations in the brucite layer changes). More information on compounds of the hydrotalcite type are contained in the review xe2x80x9cHydrotalcite-type anionic clays: preparation, properties and applicationsxe2x80x9d (Catalysis Today, 11(1991) 173-301).
Once the compound having general formula (I) has been prepared and isolated, before beginning step (2), it is preferable to subject the compound itself to a drying step in order to reduce the quantity of water (or solvent) pg,9 adsorbed.
Step (2) consists in the calcination of the compound having general formula (I) at a temperature ranging from 300 to 500xc2x0 C., preferably from 350 to 450xc2x0 C.
As already mentioned, before being used in the Fischer-Tropsch process, the catalyst is subjected to reduction. This operation is carried out, in the preferred embodiment, by means of treatment with hydrogen, optionally diluted with inert gases, for example nitrogen. The reduction step is preferably effected at a temperature ranging from 300xc2x0 C. to 500xc2x0 C., even more preferably from 320xc2x0 C. to 450xc2x0 C. It is possible to operate either under pressure or at atmospheric pressure, the latter condition being preferred. The duration of the reduction process varies in relation to the experimental conditions (temperature, pressure, dilution or non-dilution of the hydrogen).
A further object of the present invention relates to a process for the preparation of prevalently C22+ hydrocarbons, or so-called Fischer-Tropsch waxes, characterized in that it is carried out in the presence of the catalyst according to claim 1.
The Fischer-Tropsch process is the well-known reaction between CO and H2, optionally diluted with CO2 and/or N2, to give prevalently C22+ hydrocarbons.
The reaction conditions are described in literature. For example, the temperatures can range from 170xc2x0 C. to 400xc2x0 C., preferably from 180xc2x0 C. to 250xc2x0 C., whereas the pressures can vary from 1 to 100 bars, preferably from 15 to 40 bars. The CO/H2 ratio can vary from 0.5/1 to 4/1, preferably from 1.87/1 to 2.5/1, the stoichiometric value (more or less 3%) being preferred.
The catalyst of the present invention can be used either in a fixed bed reactor or in a slurry-type reactor.
The following examples are provided for a better understanding of the present invention.